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<the rest="" of="" contents="" |="" 余下全文="">

<p><strong>8.1 继承的基础知识</strong></p>
<pre><code>Java支持单继承，但不支持多继承。

尽管子类包含超类的所有成员，但是子类不能访问超类中被声明为私有的所有成员。
</code></pre><p><strong>8.2 使用super关键字</strong></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div></pre></td><td class="code"><pre><div class="line">class Box</div><div class="line">&#123;</div><div class="line">	doubel width;</div><div class="line">	doubel height;</div><div class="line">	doubel depth;</div><div class="line">	</div><div class="line">	Box(doubel w,doubel h,doubel d)</div><div class="line">	&#123;</div><div class="line">		width = w;</div><div class="line">		height = h;</div><div class="line">		depth = d;</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class BoxWeight extents Box</div><div class="line">&#123;</div><div class="line">	doubel weight;</div><div class="line">	</div><div class="line">	BoxWeight(doubel w,doubel h,doubel d,doubel m)</div><div class="line">	&#123;</div><div class="line">		width = w;  //当超类中width是私有成员时，就不能这样赋值，只能使用super()进行赋值。</div><div class="line">		height = h;</div><div class="line">		depth = d;</div><div class="line">		weight = m;</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>但是，有时候希望创建只有自己才知道实现细节的超类（也就是说将超类的数据成员保存为私有）。对于这种情况，子类就不能直接访问或初始化这些变量。
因为封装是OOP的主要特性，所以Java为这一个问题提供了一个解决的方案是很正常的。
无论何时，当子类需要引用它的直接超类时，都可以使用关键字super.

super有两种一般用法：第一种用于调用超类的构造函数；第二种用于访问超类中被子类的某个成员隐藏的成员。

(1)使用super调用超类的构造函数。

格式：super(arg-list);
其中，arg-list是超类中构造函数中需要的全部参数，并且super()总是子类的构造函数中执行的第一条语句。
例如：
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">class BoxWeight</div><div class="line">&#123;</div><div class="line">	BoxWeight(doubel w,doubel h,doubel d,doubel m)</div><div class="line">	&#123;</div><div class="line">		super(w,h,d);</div><div class="line">		weight = m;</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>其中，BoxWeight()使用参数w、h和d调用super(),这会调用Box类的构造函数，使用这些值初始化width、height和depth变量。
BoxWeight自身不在初始化这些值，而是只需要初始化自身特有的值：weight.这样一来，Box类就可以将这些变量声明为私有的。

总结：当子类调用super()时，会调用直接超类的构造函数。因此，super()总是引用调用类的直接超类。即使在多层次继承中也是如此。
      此外，super()总是子类构造函数中执行的第一句。

(2)使用super访问超类中被子类的某个成员隐藏的成员。

super的这种一种用法和this有些类似，只不过super总是引用超类。
格式：super.member
其中，member既可以是方法，也可以是实例变量。

最常用的形式情况是,子类的成员名称隐藏了超类中的同名成员。
例如：
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div></pre></td><td class="code"><pre><div class="line">class A </div><div class="line">&#123;</div><div class="line">	int i;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class B extents A</div><div class="line">&#123;</div><div class="line">	int i;</div><div class="line">	</div><div class="line">	B(int a,int b)</div><div class="line">	&#123;</div><div class="line">		super.i = a;</div><div class="line">		i = b;</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	void show()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;i in superclass:&quot;+super.i);</div><div class="line">		System.out.println(&quot;i in subclass:&quot;+i);</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class SueSuper</div><div class="line">&#123;</div><div class="line">	public static void main(String[] args)</div><div class="line">	&#123;</div><div class="line">		B subOb = new B(1,2);</div><div class="line">		subOb.show();</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>输出：
i in superclass: 1
i in subclass: 2
</code></pre><p>8.4 构造函数的调用时机</p>
<pre><code>答案是：在类层次中，从超类到子类按照继承的顺序调用构造函数。
</code></pre><p><strong>8.5 方法重写</strong></p>
<pre><code>在类层次中，如果子类的一个方法和超类的一个方法具有相同的名称和类型签名，那么称子类中的这个方法重写了超类中相应的那个方法。
当子类中调用被重写的方法时，总是调用有子类定义的版本，由超类定义的版本会被隐藏。
例如：
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div></pre></td><td class="code"><pre><div class="line">class A</div><div class="line">&#123;</div><div class="line">	int i,j;</div><div class="line">	</div><div class="line">	A(int a,int b)</div><div class="line">	&#123;</div><div class="line">		i = a;</div><div class="line">		j = b;</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	void show()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;i and j:&quot;+i+&quot; &quot;+j);</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class B extents A</div><div class="line">&#123;</div><div class="line">	int k;</div><div class="line">	</div><div class="line">	B(int a,int b,int c)</div><div class="line">	&#123;</div><div class="line">		super(a,b);</div><div class="line">		k = c;</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	void show()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;k:&quot;+k);</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class Override</div><div class="line">&#123;</div><div class="line">	public static void main(String[] args)</div><div class="line">	&#123;</div><div class="line">		B subOb = new B(1,2,3);</div><div class="line">		</div><div class="line">		subOb.show();</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>输出：
k:3
当在类型B的对象上调用show()方法时，使用的是B中定义的版本。也就是说，类B中的show()版本覆盖了在类A中声明的版本。

如果希望访问到超类中被重写的方法，可以通过super完成该操作。例如将上面的类B改成如下形式：
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line">class B extents A </div><div class="line">&#123;</div><div class="line">	int k;</div><div class="line">	</div><div class="line">	B(int a,int b,int c)</div><div class="line">	&#123;</div><div class="line">		super(a,b);</div><div class="line">		k = c;</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	void show()</div><div class="line">	&#123;</div><div class="line">		super.show(); //this calls A&apos;s show()</div><div class="line">		System.out.println(&quot;k:&quot;+k);</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>输出为：
i and j: 1,2
k:3

只有当两个方法的签名(方法名称和参数列表的顺序和类型)相同时，才发生重写。如果不相同，那么这两个方法只是简单的重载关系。
</code></pre><p><strong>8.6 动态方法调度</strong></p>
<pre><code>方法重写形成了动态方法调度(dynamic method dispatch)的基础，动态方法调度是Java中最强大的功能之一。动态方法调度是一种机制，
通过这种机制可以在运行时，而不是在编译时解析对重写方法的调用。动态方法调度很重要，因为这是Java实现运行时多态的机理所在。

首先在此声明一个重要的原则：超类引用变量可以指向子类对象。Java利用这一事实，在运行时解析对重写方法的调用。
下面是实现原理：当通过超类引用调用重写的方法时，Java根据在调用时所引用对象的类型来判断调用哪个版本的方法。
因此，这个决定是在运行时做出的。如果引用不同的对象，就会调用不同版本的重写方法。换句话说，是当前正在引用的对象
的类型（而不是引用变量的类型）决定了要执行哪个版本的重写方法。
例如：
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div></pre></td><td class="code"><pre><div class="line">class A</div><div class="line">&#123;</div><div class="line">	void callme()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside A&apos;s callme method&quot;)</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class B extents A </div><div class="line">&#123;</div><div class="line">	void callme()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside B&apos;s callme method&quot;);</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class C extents A </div><div class="line">&#123;</div><div class="line">	void callme()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside C&apos;s callme method&quot;);</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class DisPatch</div><div class="line">&#123;</div><div class="line">	public static void main(String[] args)</div><div class="line">	&#123;</div><div class="line">		A a = new A();</div><div class="line">		B b = new B();</div><div class="line">		C c = new C();</div><div class="line">		A r;</div><div class="line">		</div><div class="line">		r=a;</div><div class="line">		r.callme():</div><div class="line">		</div><div class="line">		r=b;</div><div class="line">		r.callme();</div><div class="line">		</div><div class="line">		r=c;</div><div class="line">		r.callme();</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>输出：
Inside A&apos;s callme method
Inside B&apos;s callme method
Inside C&apos;s callme method
</code></pre><p><strong>8.7 使用抽象类</strong></p>
<pre><code>下面的程序创建了一个超类Figure,该类存储二维对象的尺寸。该类还定义了一个area()方法，该方法计算对象的面积。
这个程序从Figure类派生了两个子类。第一个子类是Rectangle(矩形),第二个子类是Triangle(三角形)，每个子类都重写了area()方法，
从而可以相应的返回矩形和三角形的面积。
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div><div class="line">52</div><div class="line">53</div><div class="line">54</div><div class="line">55</div><div class="line">56</div><div class="line">57</div><div class="line">58</div><div class="line">59</div><div class="line">60</div><div class="line">61</div><div class="line">62</div><div class="line">63</div><div class="line">64</div><div class="line">65</div></pre></td><td class="code"><pre><div class="line">class Figure</div><div class="line">&#123;</div><div class="line">	double dim1;</div><div class="line">	double dim2;</div><div class="line">	</div><div class="line">	Figure(double a, double b)</div><div class="line">	&#123;</div><div class="line">		dim1 = a;</div><div class="line">		dim2 = b;</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	doubel area()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Area for Figure is undefined.&quot;)</div><div class="line">		return 0;</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class Rectangle extents Figure</div><div class="line">&#123;</div><div class="line">	Rectangle(double a, double b)</div><div class="line">	&#123;</div><div class="line">		super(a,b);</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	double area()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside Area for Rectangle.&quot;);</div><div class="line">		return dim1*dim2;</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class Triangle extents Figure</div><div class="line">&#123;</div><div class="line">	Triangle(double a,double b)</div><div class="line">	&#123;</div><div class="line">		super(a,b);</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	double area()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside Area for Triangle.&quot;);</div><div class="line">		return dim1*dim2/2;</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class FindAreas</div><div class="line">&#123;</div><div class="line">	public static void main(String[] args)</div><div class="line">	&#123;</div><div class="line">		Figure f = new Figure(10,10);</div><div class="line">		Rectangle r = new Rectangle(9,5);</div><div class="line">		Triangle t = new Triangle(10,8);</div><div class="line">		Figure figure;</div><div class="line">		</div><div class="line">		figure = f;</div><div class="line">		System.out.println(&quot;Area is&quot;+figure.area());</div><div class="line">		</div><div class="line">		figure = r;</div><div class="line">		System.out.println(&quot;Area is&quot;+figure.area());</div><div class="line">		</div><div class="line">		figure = t;</div><div class="line">		System.out.println(&quot;Area is&quot;+figure.area());</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>输出：
Area for Figure is undefined.
Area is 0
Inside Area for Rectangle.
Area is 45
Inside Area for Triangle.
Area is 40

有时候会希望创建这样一种超类——只定义被所有子类共享的一般形式，而让每个子类填充细节。这种类决定了子类必须实现的方法的本质。
例如，分析Triangle类，如果没有定义area()方法，它将没有意义。对于这种情况，你会希望有某些方式能够确保子类确实、真正的重写了所有必需的方法。
Java对这个问题提供的解决方法是抽象方法(abstract method).

可以通过abstract类型修饰符，要求特定的方法必须被子类重写。这些方法有时被称做子类责任(subclass responsibility).
因为在超类中没有提供实现。因此，子类必须重写他们————不能简单的使用在超类中定义的版本。

为了声明抽象方法，需要使用下面的一般形式：
abstract type name(parameter-list);

任何包含一个或多个抽象方法的类都必须被声明为抽象。为了声明抽象类，只需要简单的在类声明的开头、在class关键字前面使用关键字abstract。
对于抽象类不存在对象。也就是说，不能使用new运算符直接实例化抽象类。这种类的对象是无用的，因为抽象类的定义是不完整的。
此外，不能声明抽象的构造方法，也不能声明抽象的静态方法。抽象类的所有子类，要么实现超类中的所有抽象方法，要么自己也声明为抽象类。
抽象类中可以包含非抽象方法。

将上面的例子，写成抽象的形式：
</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div><div class="line">52</div><div class="line">53</div><div class="line">54</div><div class="line">55</div><div class="line">56</div><div class="line">57</div></pre></td><td class="code"><pre><div class="line">abstract class Figure</div><div class="line">&#123;</div><div class="line">	double dim1;</div><div class="line">	double dim2;</div><div class="line">	</div><div class="line">	Figure(double a,double b)</div><div class="line">	&#123;</div><div class="line">		dim1 = a;</div><div class="line">		dim2 = b;</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	abstract double area();</div><div class="line">&#125;</div><div class="line"></div><div class="line">class Rectangle extents Figure</div><div class="line">&#123;</div><div class="line">	Rectangle(double a,double b)</div><div class="line">	&#123;</div><div class="line">		super(a,b);</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	double area()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside Area for Rectangle.&quot;);</div><div class="line">		return dim1*dim2;</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class Triangle extents Figure</div><div class="line">&#123;</div><div class="line">	Triangle(double a,double b)</div><div class="line">	&#123;</div><div class="line">		super(a,b);</div><div class="line">	&#125;</div><div class="line">	</div><div class="line">	double area()</div><div class="line">	&#123;</div><div class="line">		System.out.println(&quot;Inside Area For Triangle.&quot;);</div><div class="line">		return dim1*dim2/2;</div><div class="line">	&#125;</div><div class="line">&#125;</div><div class="line"></div><div class="line">class AbstractAreas</div><div class="line">&#123;</div><div class="line">	public static void main(String[] args)</div><div class="line">	&#123;</div><div class="line">		Figure f;</div><div class="line">		Rectangle r = new Rectangle(10,10);</div><div class="line">		Triangle t = new Triangle(10,8);</div><div class="line">		</div><div class="line">		f = r;</div><div class="line">		System.out.println(&quot;Area is &quot;+f.area());</div><div class="line">		</div><div class="line">		f = t;</div><div class="line">		System.out.println(&quot;Area is &quot;+f.area());</div><div class="line">	&#125;</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<pre><code>尽管不能创建Figure类型的对象，但是可以创建Figure类型的引用变量。因为，Java运行时多态是通过使用超类引用实现的。
因此，必须能够创建指向抽象类的引用，从而可以用于指向子类对象。
</code></pre><p><strong>8.8 在继承中使用final关键字</strong></p>
<pre><code>关键字final有三个用途。
首先，可以用于创建自己命名常量的等价物。
final关键字的另外两个用途是用于继承。

(1)使用final关键字阻止重写

虽然方法重写是Java中最强大的特性之一，但是有时候希望阻止这种情况的发生。为了禁止重写方法，可以在方法中声明的开头使用final作为修饰符。
使用final声明的方法不能被重写。

经方法声明为final，有时可以提高性能：编译器可以自由的内联对这类方法的调用，因为编译器知道这些方法不能被子类重写。当调用小的fanal方法时，
Java编译器通常可以复制子例程的字节码，直接和调用方法的编译代码内联到一起，从而可以消除方法调用所需要的开销。

内联是final方法才有的一个选项。通常Java在运行时动态分析对方法的调用，这称为后期绑定。
但是，因为final方法不能被重写，所以对final方法的调用可以在编译时解析，这称为早期绑定。

(2)使用final关键字阻止继承

有时候希望阻止类的继承。为此，可以在类声明的前面使用final关键字。将类声明为final,就隐式的将类的所有方法声明为final.
因此，将类同时声明为abstract和final是非法的，因为抽象类是不完整的，它依赖子类来提供完整的实现。
</code></pre><p><strong>8.9 Object类</strong></p>
<pre><code>有一个特殊的类，即Object，该类由Java定义的。所有其他类都是Object的子类。也就是说，Object是所有其他类的超类。这意味着Object
类型的引用变量可以引用其他类型的对象。此外，因为数组也是作为类实现的，所以Object类型的变量也可以引用任何数组。

Object类定义了如下方法，这意味着所有对象都可以使用这些方法：

Object clone():        创建一个和将要复制对象完全相同的新对象。
boolean equals(Object object):        判定一个对象是否和另一个对象相等。
void finalize():        在回收不在使用的对象之前调用。
Class&lt;?&gt; getClass():        在运行时获取对象所使用的类
int hashCode():            返回与调用对象相关联的散列值。
void notify():        恢复执行在调用对象上等待的某个线程。
void notifyAll():        恢复执行在调用对象上等待的所有线程。
String toString():         返回一个描述对象的字符串。

void wait():
void wait(long milliseconds):
void wait(long milliseconds, int nanoseconds):         等待另一个线程的执行。

方法getClass()、notify()、notifyAll()以及wait()被声明为final.你可以重写其他方法。


出自：《Java 8编程参考官方教程(第9版)》
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